Afghan President Hamid Karzai said on Thursday he was considering changes to either the 2014 timetable for a presidential election or the transition of security to Afghan forces that year to avoid overburdening the country.

Fear is mounting among Afghans and foreign governments alike that the planned pullout of most NATO combat troops by the end of 2014 and an election in the same year could see the country engulfed in conflict and authorities too stretched.

Under the constitution, a vote must be held before the end of 2014 to elect on a successor to Karzai, who has to step aside after winning a second five-year term in 2009.

There is worry that shrinking NATO troop numbers and fledgling Afghan forces may not be able to ensure security following a series of bombings by the Taliban and the breakdown of peace talks between insurgents and the United States.

Karzai said he had been discussing the question with close advisers for months, but that no decision had been taken and it would not be made soon.

This is a question that I have had, and I have raised it with my inner circle, if we cannot have all that accomplished in 2014 because of the heavy agenda, he told a news conference in Kabul with NATO Secretary General Anders Fogh Rasmussen.

Can we bring either the transition and the return of international forces to 2013 so we can have the other agenda fulfilled in 2014 with less to do, or should we allow the transition process to complete itself in 2014, but bring the presidential election one year earlier to 2013.

Britain's ambassador to Afghanistan, William Patey told the Guardian newspaper this month that there are perfectly good arguments why 2013 would be a better time to hold the presidential election.

Afghan forces are increasingly taking over a lead security role and are due to take over full responsibility from NATO by the end of 2014, when most foreign combat troops will have gone.

Rasmussen said he was confident the full transition to Afghan soldiers and police would be completed by the end of 2014.

We will not abandon Afghanistan after 2014. We will stay and assist. NATO is here as Afghanistan's partner, Rasmussen said in a message on Twitter after meeting Karzai.

NATO nations have been planning to build the Afghan police and army to a force of about 352,000 and then scaling them back as - they hope - the security threat fades and as financial support shrinks from the West.

Karzai said Afghanistan would not consider reducing the number of security forces until at least 2015 or 2016 and that would depend on the situation, and the training and readiness of the armed forces.

There is growing sensitivity over the presence of foreign troops after a series on incidents, including the massacre of 17 Afghan villagers for which a U.S. soldier was charged, and the burning of copies of the Koran at a NATO base.

U.S. and Afghan officials have been trying to negotiate an accord for a long-term U.S. presence in Afghanistan beyond the 2014 deadline to allow advisers and possibly some special forces to stay on.

(Additional reporting by Mirwais Harooni; Editing by Sanjeev Miglani)